There has been considerable effort devoted to improving the sensitivity of silver halide crystals to actinic radiation and thereby increasing the sensitivity of the photographic elements in which they are contained. In this regard, photographic chemists have attempted to vary the components of, or the processes for making, silver halide emulsions. One particularly preferred means to improve sensitivity has been to chemically sensitize photographic emulsions with one or more compounds containing labile atoms of gold, sulfur, selenium, or the like. Examples of chemically sensitized photographic silver halide emulsion layers are described in, for example, Research Disclosure, Item No. 308119, December 1989, Section III, and the references listed therein. (Research Disclosure is published by Kenneth Mason Publications Ltd, Dudley Annex, 12a North Street, Emsworth, Hampshire PO10 7DQ, England.)
Many gold sensitizers have been described. For example, U.S. Pat. No. 3,503,749 describes the use of water soluble Au(I) thiolate salts comprising one Au atom ligated to one sulfur containing ligand. U.S. Pat. No. 5,220,030 teaches the use of Au(I) compounds with bis mesoionic heterocycles. U.S. Pat. No. 5,252,455 and U.S. Pat. No. 5,391,727 disclose the use of Au(I) macrocyclic cationic sensitizers. U.S. Pat. No. 5,049,484 teaches the use of Au(I) sensitizers having a Au atom ligated to the nitrogen atom of heterocyclic rings. U.S. Pat. No. 5,620,841 discloses the use of gelatin dispersions of a Au(I) thiosulfonato sensitizer with two different ligands, at least one of which is mesoionic, and U.S. Pat. No. 5,700,631 teaches the use of gelatin dispersions of Au(I) thiosulfonato sensitizers with two different ligands, at least one of which is a thioether group. JP 8069075 discusses the use of organic gold sulfide compounds in the sensitization of low-fogging and high contrast silver halide photographic materials. However, all of the above compounds have one or more disadvantages such as lack of water solubility, difficulty of synthesis, or poor stability.
An improved gold sensitizer avoiding the above difficulties has been described by Lok et al in U.S. Pat. No. 5,912,112. Although a more convenient gold source, chemical sensitizations with this material in combination with common sulfur sources such as thiosulfate do not always yield acceptable fog levels.
Other patents relating to Au(I) complexes are U.S. Pat. Nos. 5,945,270 and 6,034,249 of Lok et al.